Technical Field
Exemplary aspects of the present invention generally relate to a belt conveyor unit and an image forming apparatus, such as a copier, a facsimile machine, or a printer including the belt conveyor unit.
Description of the Related Art
There has been known a color image forming apparatus using an electrophotographic method in which toner images of different colors formed on latent image bearing members are primarily transferred onto an intermediate transfer member and then onto a recording medium in a secondary transfer process.
There are two types of secondary transfer devices employed in the image forming apparatus of this kind: a roller-transfer type and a belt-transfer type. The secondary transfer device of the roller-transfer type includes an intermediate transfer member and a transfer roller, and a recording medium is interposed between the intermediate transfer member and the transfer roller, and conveyed. The latent image is secondarily transferred onto the recording medium while the recording medium is conveyed. The secondary transfer device of the belt-transfer type includes a secondary belt entrained around and stretched taut between the rollers. A recording medium is interposed between the secondary transfer belt and the intermediate transfer member, and the latent image is secondarily transferred onto the recording medium while the recording medium is conveyed.
In the secondary transfer device of the belt-transfer type, the recording medium is interposed in a secondary transfer nip between the secondary transfer belt and the intermediate transfer member, and the recording medium is absorbed to the secondary transfer belt upstream and/or downstream from the secondary transfer nip. In this configuration, the recording medium is held and conveyed reliably, not only at the secondary transfer nip, but also at the upstream side and the downstream side in the conveyance direction of the recording medium. Thus, it is generally said that the belt-transfer type allows more reliable sheet conveyance than the roller-transfer type.
Although advantageous, similar to a generally-known belt conveyor unit, the belt transfer method may cause the secondary transfer belt to travel to one side in the width direction of the belt or repeatedly wander back and forth on either side in the width direction of the belt.
Such belt wander and belt meander are attributed to dimensional tolerances of parts constituting the secondary transfer device, for example, variations in a parallelism error of rotary shafts of the plurality of rollers that supports the secondary transfer belt, variations in an outer diameter of the rollers, and variations in the tension of the secondary transfer belt due to changes in the circumferential length of the secondary transfer belt itself.
More specifically, because of the reasons above, the secondary transfer belt does not travel linearly, but keeps traveling out of alignment in the direction of displacement of the roller shaft (i.e., the width direction of the belt). In particular, in a case in which the secondary transfer device is relatively small, since the distance between the shafts of rollers is relatively short, the dimensional tolerance of the rollers easily affects the frequency of the belt wander. Therefore, there is demand for prevention of the belt wander.
One example of a known belt alignment device includes a guide rib provided at both ends of an inner circumferential surface of the belt in the width direction of the belt. In the known belt alignment device, when the belt runs off center in the width direction of the belt, the guide ribs come into contact with the end surface of the roller around which the belt is entrained, preventing the belt from traveling further in the width direction of the belt.
Another example of the belt alignment device is a steering-roller type belt alignment device in which a steering roller which is one of a plurality of rollers around which the belt is entrained is tiltably disposed relative to the main body. The belt alignment device of this kind includes a belt deviation detector. When the belt deviation detector detects the belt wandering off its track, the steering roller is tilted by a drive force from a drive source such as a motor to move the belt in the direction opposite the direction of the belt wander.
There is a drawback in the belt alignment device using the guide rib in that the width of the secondary transfer belt needs to be wider than the width of all rollers around which the secondary transfer belt is entrained in consideration of the guide rib disposed at both ends of the belt in the width direction of the belt, complicating efforts to achieve downsizing.
However, the width of the secondary transfer belt wider than the width of the transfer roller in the axial direction thereof causes the following drawback. That is, when the secondary transfer belt passes through the secondary transfer nip, the secondary transfer belt is pushed against the edge portion of the transfer roller by the intermediate transfer member. As a result, buckling load that causes the end portion of the secondary transfer belt in the width direction thereof to buckle is applied to the secondary transfer belt. Consequently, every time the secondary transfer belt passes through the secondary transfer nip, the buckling load is applied to the secondary transfer belt, damaging the secondary transfer belt and shortening the product life cycle of the secondary transfer belt.
By contrast, the width of the secondary transfer belt of the belt alignment device using the steering roller can be shorter than the width of rollers in the axial direction because no guide ribs need to be disposed at both ends of the secondary transfer belt in the direction of the width thereof. This configuration may prevent the buckling load from being applied to the secondary transfer belt when the secondary transfer belt passes through the secondary transfer nip, thereby enhancing the product life cycle of the secondary transfer belt.
However, this configuration requires the drive source that tilts the steering roller and the space for the drive source. Furthermore, reducing the width of the secondary transfer belt simply shorter than the width of the transfer roller does not prevent the end portion of the secondary transfer belt in the width direction from running off the edge portion of the transfer roller when the belt travels out of alignment.
In view of the above, there is demand for an image forming apparatus that enhances the product life of the belt without increasing the size of the apparatus.